Sasuke's confindense has nothing to do with power he might have aqurried. he just hasn't been beaten up enough by his parents as a child, so he doesn't know his place.

domestic abuse jokes aside, Sasuke had a confidense break down only once, before the 'sasuke in a box' arc began (after getting saved by kakashi and takin' a beatin' from the sound quatro). every other fight, including when he met Itachi previously, he had the same annoying smirk on damn face